Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures), and are widely used in automobiles and the like. A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. For instance, in a centripetal turbine the turbine housing defines an annular inlet passageway around the turbine wheel and a generally cylindrical axial outlet passageway extending from the turbine wheel. Rotation of the turbine wheel drives a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.
It is also well known to provide turbochargers with a bypass passageway between the exhaust inlet and the exhaust outlet portions of the turbine housing to enable control of the turbocharger boost pressure. A wastegate valve is located in the passageway and is controlled to open the passageway when the pressure level of the boost air increases to a predetermined level, thus allowing some of the exhaust gas to bypass the turbine wheel preventing the boost pressure from rising further. The wastegate valve is generally actuated by a pneumatic actuator operated by boost air pressure delivered by the compressor wheel.
The conventional pneumatic actuator comprises a spring loaded diaphragm housed within a canister (the wastegate actuator can) which is mounted to the compressor housing. The diaphragm acts on a connecting rod which actuates the wastegate valve assembly which is mounted in the turbine housing.
The actuator can is connected to the compressor outlet by a flexible hose to deliver boost air to the can which acts on the diaphragm to oppose the spring bias. The spring is selected, and the actuator and wastegate valve initially set, so that under low boost conditions the wastegate valve remains closed. However, when the boost pressure reaches a predetermined maximum the diaphragm is moved against the action of the spring and operates to open the wastegate valve (via the connecting rod and linking arm) thereby allowing some exhaust gas to bypass the turbine wheel.
In conventional arrangements the wastegate valve is mounted on a valve stem which extends through the turbine housing and which is rotated to open and close the valve. Rotation of the valve stem is achieved by the reciprocal motion of the actuator rod (as the spring loaded diaphragm moves back and forth within the actuator canister) via a lever arm which links the end of the actuator rod to the valve stem. To accommodate the motion of the actuator rod, there is a pivotable joint between the lever arm and the actuator rod, the opposite end of the lever arm being secured (typically by welding) to the end of the valve stem. For accurate operation of the actuator it is also important that the diaphragm maintains alignment within the canister, and thus that the rod maintains its alignment along the axis of the canister. It is therefore known to design the pivotal joint between the actuator rod and the lever arm to allow a slight amount of movement along axis of the lever arm to limit the tendency of the actuator rod to be pulled off-line as it reciprocates.
It will be appreciated that the pressure at which the wastegate valve begins to open, known as the “lift off point”, is critical and must therefore must be very carefully set when the actuator and wastegate are assembled to the turbocharger. With the conventional actuator assembly described above, initial set up is achieved by a process known as “weld to set”. The actuator canister, actuating rod, and the lever arm are pre-assembled, and mounted to the turbocharger. The wastegate valve is then clamped shut from within the turbine housing and the actuator canister is pressurised to the desired lift off pressure. With the diaphragm, actuator rod, and valve thus held in their respective relative positions immediately prior to lift off, the end of the lever arm is welded to the valve stem. Accordingly, any increase in the pressure supply to the actuator above the predetermined lift off pressure will cause the valve to open.
It will be appreciated that the pressure at which the diaphragm of the actuator begins to move is dependent upon the spring rate. Because tolerances to which springs can practically be manufactured mean that there can be variations in spring rate from one spring to the next, it is conventional to employ an adjustable length actuator rod to enable the length of the rod to be adjusted prior to welding the lever arm to the valve stem to ensure that the actuator rod and diaphragm are in proper alignment with the actuator canister at the lift off point.
The steps involved in the weld to set process can therefore be summarised as: holding the valve member in a closed position; pressurising the actuator canister to the lift off pressure; adjusting the length of the actuator rod; and then welding the end of the lever arm to the valve stem.